Reflecting telescopes are known, in which the optical axis of the primary mirror is parallel to the tube or the light incidence direction. The adaptive optics with the reflecting mirror are located in the focal point in the light incidence area, which leads to shading.
In addition, Wilhelm Herschel discloses a reflecting telescope, whose rotationally symmetrical primary mirror is sloped with respect to the light incidence direction in such a way that the focal point is located outside the tube, so that the adaptive optics can be positioned outside the incidence of light. However, as a result of the mirror slope the light paths differ, which leads to astigmnatic image distortions. Thus, a correcting plate ,must be used for compensating the different light paths.